... similar problem with my 5D (mk1). When it was new out of the box the focus points would light completely nor brightly. I sent it to Canon and they fixed it. I think I had to send it twice to before it was perfect. 3 years later they still light perfectly.

Well mine is second hand from ebay so I wonder how much Canon will charge me to fix it and whether or not I should give the ebay seller positive feedback. The external condition was just as he described but he didn't mention the indicator light issue ...

Just to confirm, with the dioptre adjustment done, the AF squares should be clearly bright and very sharp, even looking at bright objects. If the SD screen is fitted backwards, it will be very blurry, and should be immeadiately obvious (everybody's obvious being different of course!).

You could send a message to the seller, asking him if the SD screen has been removed for cleaning, and if he had any issue with it. If you can see a clear problem, then so could he (probably). He might be happy to tell you of any repairs he has had done. At worst he won't reply, but generally ebayers like to get good evaluations.
P.S. you have 60 days to leave an evaluation on ebay, no more